Scanners convert hard copy analog images on a media into image signals typically in the form of digital data. The media can be either transmissive (for example, a photographic negative) or reflective (for example, paper). The use of scanners has become widespread for a variety of applications, including storing, manipulating, transmitting and displaying or printing copies of the images. For example, images captured in photographic media can be converted to digital data and stored on compact discs for readout and display as a video image or for printing with various types of color printers. In order to capture the photographic image signal, an image frame is scanned with light, such as a line of light, and the light transmitted through the image is detected, typically as three primary color light intensity signals, and then digitized. The digitized values may be formatted to a standard for video display and stored on compact disc, magnetic media, or other suitable storage. Scanners take a variety of forms and the various common aspects of film digitizing, particularly line illumination and linear CCD-based digitizers, are described in greater detail in U.S. Pat. No. 5,012,346. For example, in one common type of scanner a one-dimensional sensor (typically referenced as a line sensor or one-dimensional array) is used and the illumination source directs a line of light onto the image bearing media, which is then moved one line at a time to scan the complete image line by line. In another scanner type, a two-dimensional sensor (typically referenced as a two dimensional or area array) is used, and the illumination source illuminates the entire image at the same time, so that the complete image is scanned in a single exposure. Scanners with area arrays are simpler to construct and are often preferred. However, scanners with line sensors provide higher resolution at lower equipment cost. A sensor assembly of a typical color scanner includes both the sensor and suitable electronics, so as to provide a multi-color channel output signal representing the scanned image, with each color channel corresponding to a different spectral region (for example, red, green and blue channels, or cyan, magenta and yellow channels).
In order to obtain image signals that accurately represent a scanned image, the illumination source used in a scanner must meet certain requirements. One is that the light must be of sufficiently high intensity. While this can be obtained by using higher-powered light sources, this leads to greater heat generation with required means to control such heat, and often to a light source with a shorter life. The illumination source should also provide uniform intensity of illumination so that the signal-to-noise ratio does not vary across the extent of the image of a media being scanned. Although correction to the image signal can be used to remove the effects of such non-uniformity, doing so causes regions of low illumination to be noisier than regions of high illumination from the higher gain applied there. Additionally, since the media to be illuminated may contain defects such as scratches, it is well known that the visibility of such defects may be reduced by distributing the light onto the media at angles of incidence up to +-45 degrees. This is generally accomplished either by using a diffusing element such as ground glass or a diffuse integrating chamber with or without an optical waveguide in close proximity to the media. Many of these desirable features (such as intensity and uniformity of illumination) become more difficult to obtain in area array scanners.
It is possible to use as a light source, a broadband, white light source with appropriate filters to remove undesirable spectral components. One such known prior art arrangement is illustrated in FIG. 1. In FIG. 1, a broadband white light source, in the form of an incandescent light bulb 2, is positioned in a reflector 4 to direct light to an multi-layer interference filter 6 (sometimes referred to as dichroic filters). Interference filter 6 is constructed to reject infrared (to which a scanner sensor may be sensitive). The filter may also be designed to improve color balance by removing unwanted components of visible light. Color balance is the balance between the red, green and blue channels. Such interference filters are expensive to construct. Light from filter 6 then enters a non-imaging optic light concentrator cone 8 and through an input port 11 of a integrating chamber 50. The inside of integrating chamber 50 is made of a diffuse reflective material so that a relatively uniform beam of light leaves an exit port 12 toward a media to be scanned, then onto an area array sensor.
The above illumination system is relatively effective. However, it is relatively fixed in the sense of producing one particular output, unless one adds mechanical filter wheels or similar arrangements to provide some degree of freedom over light source control. Thus, light source control beyond color balance or total exposure, is not readily obtained.
On the other hand, U.S. Pat. No. 5,191,406 discloses a line scanner in which lines of differently colored light emitting diodes (LEDs) are used to provide lines of illumination of different colors. The relative ON times of the different colored LEDs may be adjusted to provide a desired color balance between the red, green and blue lines, which correspond to the red, green and blue color channels obtained from the sensor. Again, light source control beyond color balance, is not readily obtained from such a line scanner. U.S. Pat. No. 5,003,379 discloses a scanner in which relatively complex different shaping filter sets, are used to cause a scanner spectral sensitivity to overlap the dye absorption peaks in a negative or positive image. U.S. Pat. No. 5,099,359 discloses a scanner in which, for a given film type, color balance, IR rejection and notch filtering requirements are obtained with a single interference filter. The scanners of both '379 and '359 patents then, use different filters for different films, thereby requiring mechanical filter changing arrangements, and are limited in their ability to affect scanner spectral sensitivity by virtue of filter technology. That is, filters inherently can only subtract light obtained from a light source. Furthermore, some filters can be expensive to construct. Additionally, filtering inherently wastes power and may cause unnecessary heat generation.
It would be desirable to provide a scanner which has any one or more of the following properties, namely the scanner sensitivity can not only be controlled for color balance between color channels, but also provide a readily shaped sensitivity spectrum within one more color channels which is not limited by filter technology, which can very readily and accurately alter the shape of the scanner sensitivity spectrum for different media to be scanned without the need for cumbersome filter changes, which has relatively high power efficiency, and which can still provide good light uniformity to an image to be scanned, and which illumination source generates relatively little heat.